(S)-Pregabalin, (S)-(+)-3-(aminomethyl)-5-methylhexanoic acid, a compound having the chemical structure,
is a γ-amino butyric acid or (S)-3-isobutyl (GABA) analogue. (S)-Pregabalin has been found to activate GAD (L-glutamic acid decarboxylase). (S)-Pregabalin has a dose dependent protective effect on-seizure, and is a CNS-active compound. (S)-Pregabalin is useful in anticonvulsant therapy, due to its activation of GAD, promoting the production of GABA, one of the brain's major inhibitory neurotransmitters, which is released at 30 percent of the brains synapses.
(S)-Pregabalin has analgesic, anticonvulsant, and anxiolytic activity. (S)-Pregabalin is marketed under the name LYRICA® by Pfizer, Inc., in tablets of 25, 50, 75, 150, 200, and 300 mg doses.
The preparation of (S)-Pregabalin from 3-isbutylglutaric acid is disclosed in DRUGS OF THE FURTURE, 24 (8), 862-870 (1999), and in U.S. Pat. No. 5,616,793, and is described by the following Scheme:

Accordingly, 3-isobutylglutaric acid, compound 1, is converted into the corresponding anhydride, compound 2, by treatment with acetic anhydride. The reaction of the anhydride with NH4OH produces the glutaric acid mono-amide, compound 3, which is resolved with (R)-1-phenylethylamine, yielding the (R)-phenylethylamine salt of (R)-3-(carbamoylmethyl)-5-methylhexanoic acid, compound 3-salt. Combining the salt with an acid liberates the R enantiomer, compound 4. Finally, Hoffmann degradation with Br2/NaOH provides (S)-Pregabalin.
In the above processes, the compound 3-isobutylglutaric acid (referred to as CMH-diacid), having the following structure:
is used as a precursor for 3-carbamoylmethyl-5-methyl hexanoic acid-racemate (referred to as CMH-racemate) of formula 3, which leads to (S)-Pregabalin.
Impurities in (S)-Pregabalin, such as, CMH-diacid and others, or in any active pharmaceutical ingredient (API) are undesirable and, in extreme cases, might even be harmful to a patient being treated with a dosage form containing the API.
In addition to stability, which is a factor in the shelf life of the API, the purity of the API produced in the commercial manufacturing process is clearly a necessary condition for commercialization. Impurities introduced during commercial manufacturing processes must be limited to very small amounts, and are preferably substantially absent. For example, the ICH Q7A guidance for API manufacturers requires that process impurities be maintained below set limits by specifying the quality of raw materials, controlling process parameters, such as temperature, pressure, time, and stoichiometric ratios, and including purification steps, such as crystallization, distillation, and liquid-liquid extraction, in the manufacturing process.
The product mixture of a chemical reaction is rarely a single compound with sufficient purity to comply with pharmaceutical standards. Side products and by-products of the reaction and adjunct reagents used in the reaction will, in most cases, also be present in the product mixture. At certain stages during processing of an API, such as (S)-Pregabalin, it must be analyzed for purity, typically, by HPLC or TLC analysis, to determine if it is suitable for continued processing and, ultimately, for use in a pharmaceutical product. The API need not be absolutely pure, as absolute purity is a theoretical ideal that is typically unattainable. Rather, purity standards are set with the intention of ensuring that an API is as free of impurities as possible, and, thus, is as safe as possible for clinical use. As discussed above, in the United States, the Food and Drug Administration guidelines recommend that the amounts of some impurities be limited to less than 0.1 percent.
Generally, side products, by-products, and adjunct reagents (collectively “impurities”) are identified spectroscopically and/or with another physical method, and then associated with a peak position, such as that in a chromatogram, or a spot on a TLC plate. (Strobel p. 953, Strobel, H. A.; Heineman, W. R., Chemical Instrumentation: A Systematic Approach, 3rd ed. (Wiley & Sons: New York 1989)). Thereafter, the impurity can be identified, e.g., by its relative position in the chromatogram, where the position in a chromatogram is conventionally measured in minutes between injection of the sample on the column and elution of the particular component through the detector. The relative position in the chromatogram is known as the “retention time.”
As is known by those skilled in the art, the management of process impurities is greatly enhanced by understanding their chemical structures and synthetic pathways, and by identifying the parameters that influence the amount of impurities in the final product.
Thus, there is a need in the art for Pregabalin and (S)-Pregabalin having a low level of CMH-diacid and for a process for preparation thereof.